How could I import a game character from another .blend file in the bge during runtime?
I suggest to link groups from different files.



You have one blend file with your character. It should contain all the object that initially belong to it (physics object, armature, skin meshes, etc.). Place them all in a group that you can create an instance from it. 



In your game file you can link the group from the character file. 





Then you can instantiate the linked group. 




You can have as many instances as you like. You can link the group into any number of blend files you like. 
When you update the character file the linking files will be updated when you open them (e.g. save+load = update).
This is a good option not just for a single character but for assets too (stones, trees, houses, rocks, NPCs, GUI, HUD, Menu, vehicles ...).
Be aware this method is static. It loads on file/scene load. It does not load dynamically (in this case you need LibLoad - which should work with linking blends too).


Hint:


You can create groups from instances too. This makes linkable groups that contain instances of other groups.
When you place instances in a separate scene, you can link all groups in one step by linking the scene (rather than linking each single group). This is very useful when you have a lot of assets in one file.
Be careful with naming. Groups with the same name are supported, but you have to guess which one is the one you want.